1. Field of the Invention (Technical Field)
The present invention relates to high-sensitivity detection of contaminants in gases by optical techniques generally termed photoacoustic spectroscopy (PAS) or optoacoustic spectroscopy.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Photoacoustic spectroscopy is a known technique for high sensitivity detection of trace gases. Absorption by the target species of incident optical energy results in a transient heating of the gas. If the incident optical energy is modulated then the gas is periodically heated which creates a time-varying pressure wave or sound. The sound can be measured with a microphone.
PAS is often enhanced through the use of acoustically resonant gas cells. These cells build up sound intensity at the resonance frequencies. Depending on the type of noise dominant in the system, resonant cells can dramatically improve signal-to-noise ratios and thereby, the measurement sensitivity. Individual and combination acoustic resonance modes including longitudinal, radial, and azimuthal are often utilized. Unfortunately, resonance frequencies depend on the local speed of sound which can change with temperature and gas composition. In addition, changes in cell dimensions due to mechanical stress can change these resonance frequencies. Thus, for a practical resonance-based photoacoustic spectrometer, it is necessary to maintain the modulation frequency at the acoustic resonance frequency.
The ability to maintain the optical source modulation frequency or its harmonics on an acoustic resonance frequency of a photoacoustic cell will hereafter be referred to as resonance frequency locking. In field measurements, microphone noise is not usually a photoacoustic instrument""s sensitivity-limiting noise source. When microphone noise is not the limiting noise source, operation on an acoustic resonance enhances the signal-to-noise ratio. Optoacoustic Spectroscopy and Detection, Y-H Pao, ed. (Academic Press, New York, 1977), pps. 20-22. Because there is a 1/f dependence of the photoacoustic signal on frequency, operation at a resonance frequency represents a compromise between the enhancement, or Q, of the resonance cell and higher frequency operation. Thus, resonant operation is usually desirable provided the resonance frequency is not so high that the 1/f penalty outweighs the cavity Q. For example, a cell with a resonance at 8000 Hz with a Q of 200 would generate the same signal, all other parameters being equal, as a non-resonant cell operating at 40 Hz. In many systems, there is less background acoustic noise at 8000 Hz than at 40 Hz; resonant operation at 8000 Hz then provides a better signal-to-noise ratio.
Acoustic resonances may have narrow bandwidths. As the amplification factor (or resonance cavity Q) increases for a given frequency, the bandwidth gets narrower. Thus, as sensitivity is increased by improving the acoustic quality of the photoacoustic cell, the need for a method to maintain the acoustic modulation frequency on resonance increases proportionately. For example, a cell with a Q of 200 at 8000 Hz will have a bandwidth of 40 Hz. Thus, a change of the resonance frequency of only 20 Hz will reduce the signal by a factor of 2. A change of 20 Hz can be caused by a temperature change of less than 1 degree C. at room temperature.
The present invention achieves continuous, real-time acoustic resonance frequency locking by utilizing the behavior of the detection phase of the acoustic signal with respect to the acoustic resonance. For purposes of the specification and claims, xe2x80x9cdetection phasexe2x80x9d means phase angle of a detected signal with respect to the local oscillator reference for the lock-in amplifier (or like device) having a fixed phase relationship to the optical source modulation frequency or its harmonics. The detection phase is sensitive to the slope of the resonance response of the cell at the acoustic modulation frequency. Thus, the detection phase is qualitatively the derivative of the acoustic resonance line shape. A zero crossing of the detection phase occurs on the line center of the acoustic resonance. The value of the measured detection phase can then be used to adjust the acoustic modulation frequency so as to drive the phase back towards zero. The measured detection phase provides an error signal for the location of the cell acoustic resonance. As the acoustic resonance frequency drifts with temperature, gas composition, change in cell dimensions, etc., the acoustic modulation frequency will be continually updated and maintained to match the cell acoustic resonance frequency.
This method for acoustic resonance frequency locking can be used to equal effectiveness regardless of the method of producing photoacoustic signal. In traditional PAS the optical source radiation is amplitude modulated (AM). The modulation can be achieved by means of a mechanical chopper, a shutter, an acousto-optic modulator, or modulation of a (e.g., semiconductor) pump waveform. Other methods for achieving an amplitude modulated optical source are contemplated by and fall within the invention. In addition, Southwest Sciences, Inc. has implemented wavelength modulation spectroscopy (WMS) with PAS detection, as described in U.S. patent application Ser. No. 09/687,408. With WMS, the optical radiation source is modulated in wavelength, not amplitude (if WMS is implemented with injection current modulation of a diode laser, AM results only as a side-effect). Nevertheless, WMS produces a synchronous amplitude modulated pressure wave at the microphone. Because the present acoustic line-locking mechanism depends on features of the cell acoustic resonance and not the source of the sound, it is equally applicable to AM and WMS-based PAS.
A source of acoustic power independent of PAS generation (a speaker) has previously been used to implement an acoustic frequency locking mechanism. M. W. Sigrist and coworkers generated sound at a resonance frequency of a PAS cell with a speaker whose frequency was locked to the cell resonance via the microphone""s detection phase at the resonance frequency. G. Z. Angeli, et al., xe2x80x9cDesign and characterization of a windowless resonant photoacoustic chamber equipped with resonance locking circuitryxe2x80x9d Rev. Sci. Instrum. 62, 810 (1991). The locked resonance frequency was used to generate an amplitude modulated optical frequency for PAS generation. The optical source modulation operated at a separate cell resonance that was a constant fraction of the frequency used for resonance locking. Several disadvantages of this approach are apparent. The method requires a separate acoustic source independent of the PAS generation source. The method introduces sound at frequencies other than that where the PAS signal occurs. This sound must be attenuated in order to prevent overloading of the detection microphone. Depending on the acoustic source spectral purity, noise may be induced at the PAS detection frequency. The method relies on the PAS resonance frequency and the acoustic source generated locking frequency changing in the same way in a dynamic environment. If the cell geometry changes differently for the two frequencies (due, for example, to a mechanical stress), the frequency ratio will not be constant and the lock will be lost.
In their description, Sigrist and coworkers teach away from using phase as a mechanism for acoustic resonance frequency locking. Their teaching is based on the so-called kinetic cooling effect observed for some gases. The kinetic cooling effect introduces a phase differential for PAS signal generated from some gaseous molecules relative to the signal phase that might be produced by other types of gaseous molecules. Thus, certain combinations of molecules at specific concentration ratios where their respective PAS signal amplitudes are comparable may introduce a concentration dependent PAS signal phase shift that is independent of the phase shift caused by the acoustic modulation frequency with respect to the PAS cell acoustic resonance frequency. This teaching is not applicable to the vast majority of circumstances encountered by a trace gas detection instrument. Many applications involve the detection of a single gaseous species in a bath of another gas. Since the instrument would not be configured to detect the bath gas, only one type of gaseous molecule will contribute to the PAS signal detection phase and the instrument would be optimized for that molecule. The kinetic cooling effect is not relevant in this case. Secondly, their are many combinations of types of gaseous molecules where each type contributes to the measured detection phase in the same way. Thus, no concentration dependent phase shift is produced for those combinations and the kinetic cooling effect is not relevant. Finally, in an instrument configured for a specific gaseous molecule (i.e., PAS signal detection phase optimized for that molecule), the presence of another type of gaseous molecule which produces a different PAS signal detection phase can be accommodated without loss of resonance locking fidelity providing that it is present in low concentration or its concentration is constant.
M. W. Sigrist and coworkers also implemented a type of acoustic resonance maintenance by scanning over the cell resonance. A. Thony, et al., xe2x80x9cNew Developments in CO2-Laser Photoacoustic Monitoring of Trace Gasesxe2x80x9d, Infrared Phys. Technol. 36, 585 (1995). Their method consisted of a slow scan of several discrete steps over the cell resonance. The PAS signal vs. acoustic frequency was fit to an inverted parabolic curve and the peak of the fit used as the resonance line center. The acoustic modulation frequency was then corrected to coincide with the calculated resonance line center. This method was then repeated approximately every 20 minutes. This is not a real-time resonance frequency lock. If the cell resonance drifted during the 20 minute interval between measurements, the PAS source modulation frequency would not correspond to the cell resonance frequency. In addition, sample signal acquisition is halted during the resonance scan measurement and fitting. Continuous sample measurement is precluded in this approach. The method has utility is removing very slow drift effects, but does not provide high-fidelity resonance locking for a dynamically changing PAS sample environment. Contrast that with the present invention, whereby continual maintenance of zero detection phase with respect to the cell resonance provides a real-time error signal for perpetual operation on the cell resonance with continuous PAS sample analysis.
The present invention is of a photoacoustic spectroscopy method and apparatus for maintaining an acoustic source frequency on a sample cell resonance frequency comprising: providing an acoustic source to the sample cell to generate a photoacoustic signal, the acoustic source having a source frequency; continuously measuring detection phase of the photoacoustic signal with respect to the source frequency or a harmonic thereof; and employing the measured detection phase to provide magnitude and direction for correcting the source frequency to the resonance frequency. In the preferred embodiment, sound is generated from absorption of optical power by a species inside the sample cell, preferably amplitude or wavelength modulated optical power. The invention can thus be used with a flowing gas species. Sound may also be generated from a speaker. A metric proportional to acoustic power inside the cell (as well as to concentration of an absorption species) is measured, preferably at the acoustic source frequency or a harmonic thereof.
The invention is also of an acoustic resonance frequency locked photoacoustic spectrometer comprising: a source generating a photoacoustic signal, the source having a source frequency; and a lock-in amplifier employing a detection phase of the photoacoustic signal with respect to the source frequency or a harmonic thereof, and whereby the amplifier maintains the photoacoustic signal on a resonance frequency of a sample cell.
A primary object of the present invention is to provide for continuous, real-time, acoustic resonance frequency locking in PAS.
A primary advantage of the present invention is that it can operate at high frequencies and can be used in measurement of flowing gaseous species.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.